In the early months of 2004, the soprano Violeta Urmana was asked to stand as a presidential candidate in her native Lithuania. "The day they asked, I laughed so much," she says, dissolving into a fit of giggles in her dressing room backstage at the Royal Opera House. "It was in a period when we had some problems." The Lithuanian government was in the process of impeaching President Rolandas Paksas following allegations of links to organised crime. "I said, 'Are you kidding? I don't belong to a party, either social democrat or liberal.' 'Oh, that's better,' they said. 'But what about my singing?' 'You can sing, probably one or two times a year.' My husband couldn't sleep at night - he thought I shouldn't do it - but just for one day, I was thinking, 'Oh, for Lithuania, maybe I should.'"

In Lithuania, Urmana is a national heroine, the local girl who has made it on to the world stage. "Maybe I'm an example, something not associated with these dirty politicians. They all have dirt on their hands." She wrings her own hands in a gesture that audiences may recognise when she begins playing Lady Macbeth in Verdi's opera for the Royal Opera this week. In the end, she turned the request down - although she does think that "maybe 20 years later I could be some sort of consultant in cultural things".

A remarkable artist, Urmana possesses a huge, sexy voice capable of scything through the thickest orchestral textures and of sustaining the most rapt of pianissimos. She has become associated with some of the most complex and difficult roles in opera: Lady Macbeth, the self-lacerating heroine of Ponchieli's La Gioconda, the chameleon-like Kundry in Wagner's Parsifal. Does she have a particular fascination for these formidable women? "The first thing is whether the role fits my voice," she replies. "This kind of repertoire has strong characters, normally."

Urmana talks of her voice as if it were an entity independent of herself. It's not surprising, since the past decade has seen her public metamorphosis from mezzo to soprano. She made her UK debut at the Edinburgh festival in 1998 as Eboli in Verdi's Don Carlos. She was feral and thrilling, and few doubted at the time that a new, important mezzo had arrived. Among those who did have doubts, however, was Urmana herself, who had long been questioning whether her career was progressing in the direction she wished.

She was born in Marijampole, some 140km from Vilnius, in 1961. Her father was an engineer, her mother the director of a local museum. "They were musical, but not professionals." At the age of seven, she was given a piano. "I wanted one, I don't know why, just to play. But when I started to study I somehow lost interest." In fact, she graduated with a diploma from the Lithuanian Academy of Music in Vilnius. "I was not a bad pianist, but when I'd done my last exam, I never played again. After that, I started to study singing from zero."

Urmana experienced an epiphany at the age of 16, when she took a course in musical history. Asked to study the operas of Donizetti, she found herself listening, fascinated, to Maria Callas's recording of Lucia di Lammermoor. Other singers from the 1940s, 50s and 60s were soon among her roster of idols, including Renata Tebaldi, Zinka Milanov and Helen Traubel. "I wanted to be a soprano, and for me that was clear."

It was not clear, however, to her singing teachers in Vilnius, who were struck by the richness of her voice. "Maybe I will offend some people in Lithuania, but we don't have a good singing school there. After four years of studying soprano, they just told me, 'Oh, you have quite a dark voice, probably you're a mezzo.'" And a mezzo she temporarily, and reluctantly, became.

Becoming the soprano she believed herself to be took several more years. In 1991, she moved to Munich to work with the teacher Joseph Loibl, with whom she still studies. Her international career as a mezzo was launched in 1993, but, she says, she found roles such as Amneris in Aïda "incredibly low - it felt impossible". So she tried out one of Aïda's arias - "and there was no problem". She gravitated to what she calls parts in between, such as Wagner's Kundry, which both mezzos and sopranos can sing. The breakthrough finally came in 2000, when she was singing Azucena in Verdi's Il Trovatore at La Scala. "I felt that my voice would be damaged with this kind of repertoire. I didn't even wait a month for the change. My voice decided for me."

Gradually she shed her mezzo roles, and the UK first heard the soprano Urmana as Lady Macbeth at Edinburgh in 2003, followed in 2004 by a powerhouse recital of songs by Liszt, Strauss and Rachmaninov that floored her audience. "I started to sing lieder when I started to sing differently in opera. I had the courage to show more colours. In opera it's not always possible, because of the loud orchestra," she says - although without mentioning any maestros' names.

She also gives recitals with her husband, the tenor Alfredo Nigro, whom she met while singing in a production of Gluck's Iphigénie en Aulide at La Scala. "He was at the Academia della Scala, as a young singer. When we met, we laughed a lot, because he's Alfredo and I'm Violeta, so it's like Traviata! He's a lyric tenor, a young voice, and I'm a dramatic soprano, so it's problematic to sing together."

Now that she has found her soprano voice once more, the possibilities seem endless. She has just made her first foray into Strauss with the title role of Ariadne auf Naxos at the Met. She's about to tackle Bellini's Norma, a role primarily associated with Callas, in Dresden. Aïda beckons at La Scala later this year. Maybe, in the future, "when my voice gets bigger and heavier", there will be the big Wagner roles - she's keen to sing Isolde - and Strauss's Elektra. "My voice will decide what is good for me," she says. One suspects that the metamorphoses of Violeta Urmana are not over yet.

· Macbeth opens at the Royal Opera House, London WC2, on Saturday. Box office: 020-7304 4000.

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